A head-up display device typically comprises a projection unit which produces a light beam intended to be directed towards a combiner for the projection of images, in particular operating information of the vehicle or more generally relating to driving in the form of a virtual image situated in the field of view of the driver.
A troublesome phenomenon, which occurs in such head-up display devices or more generally in any type of projection system employing a coherent light source such as a laser, is known by the English term “speckle”. The term speckle being also currently used in the field in French, although “tavelures [marks]”, “chatoiement [sparkling]”, or again laser granularity are also spoken of.
The particular spatial and temporal coherence properties of laser light are at the origin of the speckle phenomenon. After diffusion by a naturally rough surface, the coherent light waves emitted by a laser source interfere constructively at certain points in space and destructively at others; the light spots formed are as a whole called a speckle pattern and exist in all space in which waves diffused by the surface are superimposed.
The speckle phenomenon is used in certain cases to measure the deformation of objects. However, in many applications, and in particular in projection systems using diffractive elements, the speckle phenomenon is troublesome and it is important to reduce it in order to improve the quality of the image perceived by the user.
A number of solutions have been proposed to reduce speckle, most based on the averaging of the radiation phase by placing optical elements in motion. For example, rotary diffuser systems are known. Or again, translating and/or tipping diffusers or minors, which can be moved by means of actuators or piezoelectric motors. The documents EP-A1-1 510 851 and EP-A1-1 655 636 describe this type of solution.
It will be noted that, as is moreover described in these above-mentioned patent applications, the translatory and above all rotary systems are bulky, require strict movement control and can present problems of user comfort and robustness. The bulk of the traditional speckle reduction systems are such that it limits their application in certain miniaturized optical designs.
The document US 2009/016390 describes a system with speckle reduction comprising a light emitting unit associated with a wavelength selection element the principle of which is to have a plurality of light emitters that emit at slightly different wavelengths to reduce coherence and therefore speckle. Peltier elements allow it to be ensured that each of the regions of the device emits well at the required frequency. Distortions can be introduced by means of heat generating elements of piezoelectric type used as actuators, in place of the Peltier elements.
US 2001/007510 describes a speckle reducer comprising a projection surface with diffusing particles in suspension in a fluid and in constant relative motion. The fluid can be heated with resistances or be agitated by means of a piezoelectric actuator.
In similar manner, document US 2007/058135 presents a system that comprises an electrophoretic diffuser that changes the phase of the rays, and is based on agitation of particles in suspension.